Support pretending that certain iptables commands failed.

Bug: 28362720
Test: see subsequent CL.
Change-Id: Id4db5f481bd94ae04d2a5cf7c5c97be1f000c3c9
diff --git a/server/IptablesBaseTest.h b/server/IptablesBaseTest.h
index 5843361..b8ce1e2 100644
--- a/server/IptablesBaseTest.h
+++ b/server/IptablesBaseTest.h
@@ -38,10 +38,12 @@
     void expectIptablesCommands(const std::vector<ExpectedIptablesCommands>& snippets);
     void expectIptablesRestoreCommands(const std::vector<std::string>& expectedCmds);
     void expectIptablesRestoreCommands(const ExpectedIptablesCommands& expectedCmds);
+    void setReturnValues(const std::deque<int>& returnValues);
 
 protected:
     static std::vector<std::string> sCmds;
     static ExpectedIptablesCommands sRestoreCmds;
+    static std::deque<int> sReturnValues;
     static std::deque<std::string> sPopenContents;
     static std::deque<std::string> sIptablesRestoreOutput;
     int expectIptablesCommand(IptablesTarget target, int pos, const std::string& cmd);